Black Ink-jet Inks With Reduced Lightness And Haze

ABSTRACT

A method of decreasing L*min and reducing stacked haze in a black region of an ink-jet printed image is disclosed which comprises providing an ink-jet ink receptive medium and ink-jetting at least one ink-jet ink onto the medium to form a printed medium comprising an image having at least one black region. At least one of the ink-jet inks includes a black ink-jet ink comprising a liquid vehicle and a certain black azo dye having the Formula 1. The method includes stacking the printed medium, to obtain a printed medium having decreased L*min and reduced stacked haze in the black region(s).

BACKGROUND

In ink-jet technology, the quality of high-resolution images is afunction of the ink-jet ink used to produce an image, the printer andthe print medium upon which the image is printed. In ink-jet printing,droplets of ink are ejected from a printhead in response to electricalsignals generated by a microprocessor and are deposited on a printmedium, such as paper or polymeric substrates, to form the desiredimage.

Colorant and inks for use in ink-jet printing should provide good printquality, reliability performance, and environmental robustness, whenused individually as well as in combination with other dyes and inks. Asingle ink-jet colorant and/or ink that has good chroma, gamut, hueangle, and environmental robustness (e.g., air-fastness, light-fastness,water-fastness) is not always optimal for use with other colorantsand/or ingredients of that ink, or other inks with which it is used incombination. In other words, not only does an individual colorant and/orink (e.g., cyan, magenta, or yellow ink), have to independently haveacceptable color qualities, but it should also work well when used aspart of a dye and/or ink set. Additionally, although many inks are knownwhich possess one or more desirable properties, a drawback of many inksis that improvement in one property often results in degradation inanother desirable property.

Three-ink dye based printing systems comprising cyan, magenta and yellowinks commonly suffer from high lightness (L*min) in the composite blackregions of images, resulting in poor image quality. The issue is oftencompounded by hazing of prints on special porous photo media, sometimesreferred to as “stacked haze.” This type of haze occurs after printing,as a result of stacking of printed images. All of the stacked images,other than the uppermost one which is exposed to air and dries quickly,display a hazy film on the printed images. The hazing effect canincrease the lightness of the black regions, thus reducing the sharpnessof images. Use of a dye-based black ink together with the three-inkcomposite black, to make a four-ink system, can help to improve thelightness of the black regions. However, the choice of the colorant(i.e., the selected chemical dye compounds) determines the lightness,permanence to light and ozone as well as the extent of observed haze onthe images. While some black dyes, or a combination of black dyes, ascolorants in black inks have been shown to reduce the lightness of blackregions in images, they do not address the issue of hazing.

There is continuing interest in developing colorants and black inks thatimprove image quality while maintaining permanence, especially on porousprint media.

SUMMARY

Methods of producing ink-jetted images using certain black azo dyes anddye blends that reduce both lightness of black regions and stacked hazeon printed media are disclosed, together with the printed products. Inaccordance with certain embodiments, a method of decreasing L*min andreducing stacked haze in a black region of an ink-jet printed image isprovided which comprises ink-jetting at least one ink-jet ink onto anink-jet ink receptive medium to form a printed medium comprising animage having at least one black region; and stacking the printed medium,to obtain a printed medium having decreased L*min and reduced stackedhaze in said at least one black region, relative to a printed mediumprinted with an ink-jet ink lacking said at least one black azo dye. Atleast one said ink-jet ink includes a black ink-jet ink comprising: aliquid vehicle, and at least one black azo dye having the formula:

wherein X is SO₃Q; R is unsubstituted C₁₋₄-alkyl; Q is K⁺, Na⁺, Li⁺, orany combination of those; m is 1 or 2; and n is 1, 2, or 3. Thus,embodiments described herein comprise a combination of features andadvantages intended to address various shortcomings associated withcertain prior methods. The various characteristics described above, aswell as other features, will be readily apparent to those skilled in theart upon reading the following detailed description of the preferredembodiments.

DETAILED DESCRIPTION

Certain terms are used throughout the following description and claimsto refer to particular system components. As one skilled in the art willappreciate, computer companies may refer to a component by differentnames. This document does not intend to distinguish between componentsthat differ in name but not function.

In the following discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “comprising, but not limited to . . . . ” Likewise,the singular forms “a,” “an,” and “the” include plural referents unlessthe context clearly dictates otherwise. Thus, for example, reference to“a dye” includes reference to one or more of such materials.

An “ink” or “ink-jet ink” refers to a liquid solution or dispersioncomposition that can comprise a liquid vehicle and a colorant, e.g., adye, or combination of dyes, or a pigment or combination of pigments, ora dispersed dye or combination of dispersed dyes, or a dye with apigment or a dispersed dye, or any combination of dyes and/or pigmentsand/or dispersed dyes. The liquid vehicle can be configured to be stablewith the dye through a broad range of solution characteristics, and canbe configured for ink-jet printing.

As used herein, “liquid vehicle” is defined to include liquidcompositions that can be used to carry colorants to a substrate. Liquidvehicles are well known in the art, and a wide variety of ink vehiclesmay be used in accordance with various embodiments of the presentinvention. Such ink vehicles may include a mixture of a variety ofdifferent agents, including without limitation, surfactants, solvents,co-solvents, buffers, biocides, viscosity modifiers, sequesteringagents, stabilizing agents, and water. The liquid vehicle can also carryother additives such as polymers, UV curable materials, and/orplasticizers in some embodiments.

“Media substrate” or “substrate” includes any substrate that can receiveink thereon, and can include papers, overhead projector plastics orfilms, coated papers such as photobase, fabric, art paper such as watercolor paper, optical disks, or the like.

“Porous medium” refers to any substantially inorganicparticulate-containing coated medium having surface voids and/orcavities capable of absorbing the ink-jet inks in accordance withembodiments of the present invention. Typically, porous media include asubstrate and a porous ink-receiving layer. As ink is printed on theporous media, the ink fills the voids and the outermost surface canbecome dry to the touch more quickly than in the case of traditional orswellable media.

“Stacked haze” or “stacking haze” refers to the hazing of prints onspecial porous photo media which typically occurs after printing, as aresult of stacking of printed images. It usually appears as a hazy filmon the dark regions of printed images. This hazing effect increases thelightness of black regions of an image and reduces the sharpness of theimage. Typically only the uppermost of a group of stacked images, whichis exposed to air and dries quickly, is unaffected by stacking haze.

The term “L*min” refers to the minimum lightness of a black region of animage. When the L*min indicates high lightness of a black region, thistypically results in poor image quality. Reduction of the L*min leads toa blacker black region (i.e., lower or reduced lightness).

The term “about” when referring to a numerical value or range isintended to encompass the values resulting from experimental error thatcan occur when taking measurements.

Concentrations, amounts, measurements, and other numerical data may bepresented herein in a range format. It is to be understood that suchrange format is used merely for convenience and brevity and should beinterpreted flexibly to include not only the numerical values explicitlyrecited as the limits of the range, but also to include all theindividual numerical values or sub-ranges encompassed within that rangeas if each numerical value and sub-range is explicitly recited. Forexample, a weight range of about 0.1 wt % to about 10 wt % should beinterpreted to include not only the explicitly recited concentrationlimits of 0.1 wt % to about 10 wt %, but also to include individualconcentrations such as 2 wt %, 3 wt %, 4 wt %, and sub-ranges such as 1wt % to 5 wt %, 2 wt % to 4 wt %, etc. This same principle applies toranges reciting only one numerical value. For example, a range recitedas “less than about 5 wt %” should be interpreted to include all valuesand sub-ranges between 0 wt % and 5 wt %. Furthermore, such aninterpretation should apply regardless of the breadth of the range orthe characteristics being described.

Ink-jet Ink compositions for use in inkjet printing (e.g., either orboth piezoelectric and thermal inkjet apparatus), methods for formingprinted images, and printed images are disclosed. The present inksgenerally include a specific type of black azo dye, or a blend of suchdyes, suspended in a liquid vehicle, which impart desirablecharacteristics of low lightness at the darkest regions of an image(L*min) and low haze upon stacking of the printed images. The black azodye(s) have the following formula:

wherein X is SO₃O; R is unsubstituted C₁₋₄ alkyl; Q is K⁺, Na⁺, Li⁺, orany combination of those, or Q is any other suitable monovalent cation;m is 1 or 2; and n is 1, 2, or 3. One representative member of this dyefamily is “Experimental Black 1” (1H-Pyrazole-3-carboxylic acid,4-[[6-[[4-[(1,5-disulfo-2-naphthalenyl)azo]-2-methoxy-5-methylphenyl]azo]-5-hydroxy-7-sulfo-2-naphthalenyl]azo]-4,5-dihydro-5-oxo-1-(4-sulfophenyl)-,lithium sodium salt).

Ink Compositions

The above-described black azo dyes belonging to the Formula 1 family ofcolorants are suitable for use in black inks and multi-color ink setsfor ink-jet printing on a variety of printable media, includingswellable media, porous media, coated media, and the like. The inkcompositions may be prepared in an aqueous formulation or liquid vehiclethat contain some or all of the following: water, additional colorants,co-solvents, surfactants, buffering agents, bleed control agents,biocides, sequestering agents, viscosity modifiers, humectants, binders,and any other known ink-jet ink additives.

For instance, the liquid vehicle for the ink may comprise anaqueous-based vehicle that includes water or a mixture of water and atleast one water-soluble organic solvent. Selection of a suitable mixturedepends on requirements of the specific application, such as desiredsurface tension and viscosity, the selected colorant, drying time of theliquid, and the type of substrate onto which the liquid will be printed.Some water-soluble organic solvents that may be selected for use in thepresent inks include, but are not limited to, (1) alcohols, such asmethyl alcohol, ethyl alcohol, n-propyl alcohol, iso-propyl alcohol,n-butyl alcohol, sec-butyl alcohol, t-butyl alcohol, iso-butyl alcohol,furfuryl alcohol, and tetrahydrofurfuryl alcohol; (2) ketones orketoalcohols such as acetone, methyl ethyl ketone and diacetone alcohol;(3) ethers, such as tetrahydrofuran and dioxane; (4) esters, such asethyl acetate, ethyl lactate, ethylene carbonate and propylenecarbonate; (5) polyhydric alcohols, such as ethylene glycol, diethyleneglycol, triethylene glycol, propylene glycol, tetraethylene glycol,polyethylene glycol, glycerol, 2-methyl-2,4-pentanediol1,2,6-hexanetriol and thiodiglycol; (6) lower alkyl mono- or di-ethersderived from alkylene glycols, such as ethylene glycol mono-methyl (or-ethyl)ether, diethylene glycol mono-methyl (or -ethyl)ether, propyleneglycol mono-methyl (or -ethyl)ether, triethylene glycol mono-methyl (or-ethyl)ether and diethylene glycol di-methyl (or -ethyl)ether; (7)nitrogen containing cyclic compounds, such as pyrrolidone,N-methyl-2-pyrrolidone, and 1,3-dimethyl-2-imidazolidinone; and (8)sulfur-containing compounds such as dimethyl sulfoxide andtetramethylene sulfone.

One or more solvent or co-solvents may be included in the inkformulation in a total amount generally ranging from about 1% (wt %) toabout 25%. Suitable co-solvents include, but are not limited to, watersoluble organic co-solvents, aliphatic alcohols, aromatic alcohols,diols, glycol ethers, poly(glycol) ethers, N-alkyl caprolactams,unsubstituted caprolactams, both substituted and unsubstitutedformamides, both substituted and unsubstituted acetamides, long chainalcohols, ethylene glycol, propylene glycol, diethylene glycols,triethylene glycols, glycerin, dipropylene glycols, glycol butyl ethers,polyethylene glycols, polypropylene glycols, amides, ethers, carboxylicacids, esters, organosulfides, organosulfoxides, sulfones, alcoholderivatives, carbitol, butyl carbitol, cellosolve, ether derivatives,amino alcohols, and ketones. For example, co-solvents can includeprimary aliphatic alcohols of 30 carbons or less, primary aromaticalcohols of 30 carbons or less, secondary aliphatic alcohols of 30carbons or less, secondary aromatic alcohols of 30 carbons or less,1,2-diols of 30 carbons or less, 1,3-diols of 30 carbons or less,1,5-diols of 30 carbons or less, ethylene glycol alkyl ethers, propyleneglycol alkyl ethers, poly(ethylene glycol) alkyl ethers, higher homologsof poly(ethylene glycol) alkyl ethers, poly(propylene glycol) alkylethers, higher homologs of poly(propylene glycol) alkyl ethers, and thelike. Specific examples of co-solvents that are preferably employed inthe practice of this invention include, but are not limited to,1,6-hexanediol, 1,5-pentanediol, 2-pyrrolidinone,1,(2-hydroxyethyl)-2-pyrrolidinone, trimethylolpropane (EHPD) andtetraethylene glycol.

Selected co-solvents can be added to reduce or enhance the rate ofevaporation of water in the ink-jet to minimize clogging or otherproperties of the ink such as viscosity, pH, surface tension, opticaldensity, and print quality, as desired. Multiple co-solvents can also beused, as is known in the art. An example of an ink vehicle formulationusable in formulating an ink includes one or more solvent or co-solvent,in a (total) amount in the range of about 1% (wt %) to about 50%; fromabout 2 to about 45 wt %, or from about 5 to about 35 wt %.

In some embodiments, bleed control agents are employed in an amountcomprising up to about 5 wt % of the ink-jet ink composition. Somesuitable bleed control agents consist of multivalent salts such ascalcium nitrate, calcium chloride, calcium acetate, magnesium nitrate,magnesium chloride, magnesium acetate, and combinations of any of thoseor other salts.

Various buffering agents or pH adjusting agents may also be selected forinclusion in the ink-jet ink compositions. Typical buffering agentsinclude such pH control solutions as Trizma Base, available fromSigma-Aldrich Corp. (Milwaukee, Wis.); 4-morpholine ethane sulfonic acid(“MES”); 4-morpholinepropanesulfonic acid (“MOPS”); andbeta-hydroxy-4-morpholinepropane-sulfonic acid (“MOPSO”)); hydroxides ofalkali metals and amines, such as lithium hydroxide, sodium hydroxide,potassium hydroxide; citric acid, nitric acid, hydrochloric acid, aceticacid, sulfuric acid; amines such as triethanolamine, diethanolamine, anddimethylethanolamine and other basic or acidic components. If used,buffering agents, typically comprise up to about 10 wt % of the ink-jetink composition.

Various types of additives, may be employed in the inks to optimize theproperties of the ink compositions for specific applications. Theremainder of the ink composition may be mostly water; however, otherindependently selected components may be included in some embodiments,including: surfactants, humectants, anti-kogation additives,anti-corrosive additives, polymers, preservatives, biocides that inhibitgrowth of microorganisms (such as the preservative PROXEL™ GXL availablefrom Avecia Incorporated); chelating agents (or sequestering agents)such as EDTA that eliminate deleterious effects of heavy metalimpurities; and viscosity modifiers, which may be added to improvevarious properties of the ink composition.

One or more surfactants may be included in an ink formulation, suchsuitable alkyl polyethylene oxides, alkyl phenyl polyethylene oxides,polyethylene oxide block copolymers, acetylenic polyethylene oxides,polyethylene oxide (di)esters, polyethylene oxide amines, protonatedpolyethylene oxide amines, protonated polyethylene oxide amides,dimethicone copolyols, substituted amine oxides, and the like, as wellas fluorocarbon surfactants. Examples of suitable fluorocarbonsurfactants include, but are not limited to, those commerciallyavailable from companies such as Omnova Solutions, Inc. of Fairlawn,Ohio under the trade name of PolyFox® fluorocarbon surfactants.Exemplary PolyFox® surfactants include PolyFox PF-136A, PolyFox PF-151N,PolyFox PF-154N, PolyFox PF-156A, and PolyFox PF-159.

Examples of nonionic and amphoteric surfactants include TERGITOL®compounds, which are alkyl polyethylene oxides available from DowChemical (Midland, Mich.); TRITON® compounds, which are alkyl phenylpolyethylene oxide surfactants available from Rohm & Haas Co.(Philadelphia, Pa.); BRIJ® compounds available from ICI Americas(Wilmington, Del.); PLURONIC® compounds, which are polyethyleneoxide/polypropylene oxide block copolymers; SURFYNOL® compounds, whichare acetylenic polyethylene oxides available from Air Products(Allentown, Pa.); anionic surfactants such as members of the DOWFAX™family of diphenyl sulfonate derivatives available from the Dow ChemicalCompany, and the CRODAFOS™ family of phosphate esters available fromCroda Incorporated; polyethylene oxide (“POE”) esters; POE diesters; POEamines; POE amides; and dimethicone copolyols.

The pH of the ink is adjusted, as necessary with any of a wide varietyof known pH adjustors such as potassium hydroxide or nitric acid,according to the specific pH requirements of the ink.

Producing Printed Media with Reduced L*m and Reduced Stacking Haze

An above-described black ink-jet ink may be incorporated into any typeof ink-jet material dispenser or printer, including but not limited to,thermally actuated ink-jet dispensers, mechanically actuated ink-jetdispensers, electrostatically actuated ink-jet dispensers, magneticallyactuated dispensers, piezoelectrically actuated dispensers, continuousink-jet dispensers, and similar ink-jetting devices. Any type ofsuitable ink-jet ink receptive porous media as are known for ink-jetprinting may be used to receive ink. For example, the inks may be usedfor imaging and photo printers, including home and commercial printersor for printing on inorganic porous particulate coated media (e.g.,silica and/or alumina coated media). Exemplary print media that can beused includes, but is not limited to, HP Advanced Photo Paper, HPEveryday Photo Paper, Semi-gloss, HP Photo Quality Inkjet paper, GlossyHP Photo Paper, HP Brochure Tri-fold Gloss, and HP Advanced Photo PaperSoft/High Gloss.

A process for producing printed media includes ink-jetting onto asuitable ink-jet ink receptive porous print medium an above-describedink-jet ink that contains a black azo dye of the Formula 1 family ofcolorants. In preferred embodiments, desirably low L*min in blackregions of the printed media are thereby obtained. Additionally, inpreferred embodiments, when the resulting printed media are stackedafter printing, before they are completely dried, the stacked prints arefree of stacked haze, in contrast to other similarly printed mediaproduced by printing methods which use other black inks.

A black ink may include about 2% to about 5% of a black azo dye ofFormula 1, by weight of the ink. The black azo dye imparts to at leastone black region of the resulting printed medium a reduction in stackedhaze and a lower L*min, relative to the stacked haze and L*min of thecorresponding region(s) of an image printed with the same ink-jet inklacking the black azo dye.

In some instances, printing is performed using only an above-describedblack ink, In some instances, a printing method uses a black inkcomprising a blend of an above-described black azo dye of Formula 1 andone or more other black or color dye. In still other instances, aprinting method includes ink-jetting multiple inks to create a compositeimage, using an ink set that includes an above-described black azo inkof Formula 1 and one or more color inks such as cyan, magenta andyellow.

Embodiments of printing methods employing the ink formulations describedherein potentially reduce lightness (L*min) of black regions of printedimages and at the same time reduce stacking haze, to provide improvedstacking performance of the printed media compared to many other ink-jetprinted media in common use today. For example, a black azo dye of theFormula 1 family may be employed in a single ink printing system, afour-ink, six-ink, or eight-ink system to deter the occurrence ofstacked haze and lower L*min of black regions of the printed image,compared to images printed with the same inks without the black azo dye.In preferred embodiments, the printing method yields a printed medium inwhich the L*min of at least one black region of the printed marks orimage is between about 4-12 on a CIELAB color space scale of 0-100.

In certain embodiments, the medium comprises a porous medium. In certainembodiments, the black azo dye is about 0.1% to about 10% (by weight) ofthe black ink. In some embodiments, the black azo dye is about 1% toabout 5% of the black ink. In certain embodiments, one black azo dye is1H-Pyrazole-3-carboxylic acid,4-[[6-[[4-[(1,5-disulfo-2-naphthalenyl)azo]-2-methoxy-5-methylphenyl]azo]-5-hydroxy-7-sulfo-2-naphthalenyl]azo]-4,5-dihydro-5-oxo-1-(4-sulfophenyl),lithium sodium salt.

In certain embodiments, the black ink comprises at least one Formula 1black azo dye blended with at least one other dye selected from thegroup consisting of black dyes, cyan dyes, magenta dyes and yellow dyes.In some embodiments, the cyan dye is a member of the family of dyeshaving the formula

wherein M represents a hydrogen atom or a metal atom (or an oxide,hydroxide, or halide thereof), and P_(c) represents a phthalocyaninenucleus. R¹ and R² each independently represent a substituent selectedfrom the group of —SOX¹, —SO₂X¹, SO₂NX²X³, —SO₃X⁴, wherein X¹, X², X³,and X⁴ each independently represents a hydrogen atom, a substituted orunsubstituted alkyl group, a substituted or unsubstituted aryl group, ora substituted or unsubstituted heterocyclic group. At least one of R¹ orR² has an ionic hydrophilic group as a substituent. Additionally, k andl each independently represents an integer ranging from 1 to 3.Typically, k and l are each independently selected such that k+l equals4. In some embodiments R¹ is —SO—(CH₂)₃—SO₃Z or —SO₂—(CH₂)₃—SO₃Z, and/orR² is —SO₂—(CH₂)₃—SO₂NH—C₂H₄₀C₂H₄OH or —SO₂—(CH₂)₃—SO₂NH—CH₂CH(OH)CH₃,In those embodiments, Z is lithium, sodium, potassium, ammonium,tetramethylammonium, or a mixture thereof. In some embodiments, Z islithium or potassium. In some embodiments, Z is lithium. In one specificembodiment, R¹ is —SO₂—(CH₂)₃—SO₃Z, R² is—SO₂—(CH₂)₃—SO₂NH—CH₂CH(OH)CH₃, k is 3, and Z is lithium.

Table I below, provides exemplary phthalocyanine dyes in accordance withcertain embodiments, wherein the substituents R¹ and R² are eachintroduced at the β-position. However, it should be appreciated by thoseskilled in the art, that although the preferred cyan dye has the Rsubstituent at the β-position, the dye and the ink containing the samecan further include the same basic nucleus with the R substituent atdifferent positions such as the α-position.

In the exemplary dyes shown in Table I, M is copper (Cu).

TABLE I Cyan Dye R₁ k R₂ l C1 —SO—(CH₂)₃—SO₃Li 3—SO₂—(CH₂)₃—SO₂NH—CH₂CH(OH)CH₃ 1 C2 —SO₂—(CH2)₃—SO₃K 2—SO₂—(CH₂)₃—SO₂NH—C₂H₄OC₂H₄OH 2 C3 —SO₂—(CH2)₃—SO₃K 3—SO₂—(CH₂)₃—SO₂NH—C₂H₄OC₂H₄OH 1 C4 —SO₂—(CH2)₃—SO₃Li 2.7—SO₂—(CH₂)₃—SO₂NH—CH₂CH(OH)CH₃ 1.3 C5 —SO₂—(CH2)₃—SO₃Li 2—SO₂—(CH₂)₃—SO₂NH—CH₂CH(OH)CH₃ 2In some embodiments the cyan dye of Formula 2, R¹ is —SO₂—(CH₂)₃—SO₃Z,R² is —SO₂—(CH₂)₃—SO₂NH—CH₂CH(OH)CH₃, k is 3, and Z is lithium,Experimental Cyan 1

In certain embodiments, a printing method employs an ink set thatincludes an above-described black ink and at least one color inkselected from the group consisting of cyan, magenta and yellow. In someembodiments, the ink set includes a cyan ink comprising anabove-described cyan dye. In some embodiments, a composite image thatcomprises at least one black region is formed on the medium. In someembodiments, one or more black azo dye of Formula 1 imparts to the blackregion(s) a reduction in stacked haze and a lower L*min, relative to thestacked haze and L*min of the corresponding region(s) of an imageprinted with the same ink(s) lacking the black azo dye.

EXAMPLES Example 1

A black ink was prepared by combining

2-5% of the azo dye “Experimental Black 1” (1H-Pyrazole-3-carboxylicacid,4-[[6-[[4-[(1,5-disulfo-2-naphthalenyl)azo]-2-methoxy-5-methylphenyl]azo]-5-hydroxy-7-sulfo-2-naphthalenyl]azo]-4,5-dihydro-5-oxo-1-(4-sulfophenyl)-,lithium sodium salt), which is a member of the Formula 1 family of blackazo dyes;

with a vehicle having the following ingredients:

-   -   6-12% trimethylolpropane        (2-ethyl-2-(hydroxymethyl)-1,3-propanediol, EHPD),    -   5-10% 2-pyrrolidinone,    -   1-8% 1.5 pentanediol,    -   0.1-3% Tergitol 15-S-7,    -   0.1-1% MES Acid,    -   0.02-0.1% Dowfax 8390, and    -   0.05-0.2% EDTA.        When this black ink was printed alone using a HP Deskjet 6540        printer on HP Advanced photopaper the L*min with this ink was        determined to be between 4-12.

This black ink was also printed in a 4-ink ink set with a cyan inkcontaining cuprate(3-),[[3,3′,3″-[[23-[[3-[[(2-hydroxypropyl)amino]sulfonyl]propyl]sulfonyl]-29H,31H-phthalocyanine-2,9,16-triyl-κN29,κN30, κN31, κN32]tris(sulfonyl)]tris[1-propanesulfonato]](5-)]-,trilithium, (SP-4-2)-(9Cl), referred to herein as “Experimental Cyan 1,”which is a member of the Formula 2 family of cyan dyes; a magenta inkcontaining the magenta dye 6-Benzothiazolesulfonic acid,2-[4-cyano-3-(1,1-dimethylethyl)-5-[[4-methyl-6-[(6-sulfo-2-benzothiazolyl)(2,4,6-trimethyl-3-sulfophenyl)amino]-2-[(2,4,6-trimethyl-3-sulfophenyl)amino]-3-pyridinyl]azo]-1H-pyrazol-1-yl],and a yellow ink containing a yellow dye from the following family ofdyes:

wherein R is hydrogen or an alkyl having from 1 to 6 carbon atoms; M ishydrogen or a metal atom or ammonium optionally substituted by alkyl,alkoxyalkyl or hydroxyalkyl; R₁₇, R₁₈ are independently hydrogen, alkylor alkoxy, each having from 1 to 3 carbon atoms; p₄ is from 1 to 3 andthe sulfo group is in position 1, 4, 5, 6, 7 or 8 in the case where p₄is equal to 1; the sulfo groups are in positions 4 and 8, 5 and 7, 6 and8 or 1 and 5 in the case where p₄ is equal to 2; or the sulfo groups arein positions 3, 6 and 8 or 4, 6 and 8 in the case where p₄ is equal to3; and n, m are each independently from 2 to 6. Specifically, a yellowink containing the dye, 1,3-Naphthalenedisulfonic acid,7-[[4-[[4,6-bis[(3-sulfopropyl)thio]-1,3,5-triazin-2-yl]amino]-3-methoxyphenyl]azo]-,tetrasodium salt (Yellow dye #1)

The resulting printed images had little or no haze in the dark regionsof the images after stacking of the images. Stacked haze is determinedby measuring the change in L* compared to an un-stacked control.

Example 2

Another black ink was prepared by combining

2-5% of Experimental Black 1 and 0.1-2% of a yellow dye of the Formula 3family of yellow dyes, 1,5-Naphthalenedisulfonic acid,3-[[4-[[4,6-bis[(3-sulfopropyl)thio]-1,3,5-triazin-2-yl]amino]-5-methoxy-2-methylphenyl]azo]-,tetrasodium salt (9Cl), (Yellow dye #2) with the vehicle described inExample 1.

The resulting ink was printed as described in Example 1. The L*min ofthe darkest (i.e., black,) regions of the printed images was determinedto be between 4-12. When this black ink was printed in combination witha 3-ink ink set (described in Example 1), the resulting images hadlittle or no haze in the darkest regions after stacking of the printedsheets.

Example 3

A black ink was prepared by combining

1-4% of Experimental black 1,

0.1-0.9% of the magenta dye (6-Benzothiazolesulfonic acid,2-[4-cyano-3-(1,1-dimethylethyl)-5-[[4-methyl-6-[(6-sulfo-2-benzothiazolyl)(2,4,6-trimethyl-3-sulfophenyl)amino]-2-[(2,4,6-trimethyl-3-sulfophenyl)amino]-3-pyridinyl]azo]-1H-pyrazol-1-yl],

0.5-2.5% of Experimental cyan 1, and

1.0-2.5% of Yellow dye #2 with the same vehicle as described inExample 1. Again, the L*min with this ink was determined to be between4-12 when printed as described in the above embodiment. When this inkwas printed in a 4-ink ink set (cyan, magenta, yellow, black), theresulting images had little or no haze in the darkest regions afterstacking of the printed sheets

Although 1-5% (by weight) of the black dye is the concentration rangeused in these examples, it should be understood that any concentrationrange of the black dye that provides the stated results could be usedinstead. For instance, a dye is generally present in an ink-jet ink inan amount ranging from about 0.1 wt % to about 10 wt %. The resultsobtained with the thermal ink-jet printer described above are consideredrepresentative of similar decreased lightness and reduced haze that maybe obtained with other ink-jet dispensing devices such as piezoelectricprinters, for instance.

Comparative Example A

For comparison, a black ink was prepared by combining 2-5% of the azodye Experimental black 2 belonging to the following dye class:

wherein R¹ and R² each independently represents a hydrogen, a halogen, acyano group, a carboxy group, a sulfo group, a sulfamoyl group, aN-alkylaminosulfonyl group, and a N-phenylaminosulfonyl group; R³, R⁴,R⁵, R⁶, R⁷, and R⁸ each independently represents a hydrogen, a halogengroup, a hydroxy group, a cyano group, a carboxy group, a sulfo group, asulfamoyl group, a N-alkylaminosulfonyl group, a N-phenylaminosulfonylgroup; and n is 0 or 1.

The vehicle composition was the same as in Example 1. Upon printing, asdescribed in Example 1, the darkest regions of the resulting images hadsevere hazing caused by this dye, and yielded a poorer (i.e., higher)L*min than the black dye and dye blends of Examples 1-3.

Comparative Example B

In another comparative test, a black ink was prepared by combining 2-5%of the azo dye Ilford K1334, in the same manner as Comparative ExampleA. Upon printing, it was also apparent that this dye caused the darkestregions of the resulting images to have severe hazing and the L*min washigh in stacked images.

Comparative Example C

In another comparative test, a black ink was prepared by combining 2-5%of the Experimental black 3 dye (2,7-Naphthalenedisulfonic acid,3-[[4-[[4-[[5-(aminocarbonyl)-1-ethyl-1,6-dihydro-2-hydroxy-4-methyl-6-oxo-3-pyridinyl]azo]-2-sulfophenyl]azo]-2,5-bis(2-hydroxyethoxy)phenyl]azo]-4,5-dihydroxy-,potassium sodium salt), in the same manner as Comparative Example A.Upon printing, this dye also caused the darkest regions of the resultingimages to have severe hazing and the L*min was significantly higher instacked images compared to unstacked images.

Comparative Example D

The comparative dyes Experimental black 3 and Ilford dye K1334 were alsoblended with the various yellow, cyan and magenta dyes as in Examples1-3 and tested for stacked haze and L*min. The test results showed thatthese comparative black dyes, in blends similar to those of Examples1-3, also caused severe hazing and poor L*min.

The well known CIELAB (CIEL*a*b*) system is used to measure or specifythe chromaticity (c*, square root of the sum of a*2 and b*2) or theproperties of hue [arctan (b*/a*)] and saturation (c*/L*) on atwo-dimensional chromaticity diagram. The a* measures redness-greennesson the x-axis, or the horizontal axis, and b* measuresyellowness-blueness on the y-axis, or the vertical axis. The L* measureslightness-darkness on the z-axis. To assess the L*min of the inkcompositions and the ability of the black azo dyes of Formula 1 todecrease L*min compared to other black dyes, the ink compositions wereprinted on porous media using an ink-jet printer, and the darkness ofthe black regions was assessed using the CIELAB system.

All the gray ramps were printed with the same set of cyan, magenta andyellow inks and only the black ink was varied in the experiment.

The extent of hazing in the black regions of the printed images wasassessed by measuring the average change in L* across a gray ramp or thechange in L*min of images before and after stacking of print samples.Images were printed on porous photo media and stacked for 24 hrs andthen left unstacked for a couple of hours. The L*min of the stackedimages was compared with the L*min of the uppermost unstacked image.These results are summarized in Table 1.

TABLE 1 Av L* of darkest 4 Av L* of darkest 4 Δ L* (Change in squares ina gray squares in a gray Average L*after Black ink 4-ink L*min rampbefore stacking ramp after stacking stacking) Comparative Example B 9.0913.44 18.24 4.8 Comparative Example C 9.42 13.42 18.08 4.66 Example 18.95 14.14 16.67 2.53

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. It is intended that the followingclaims be interpreted to embrace all such variations and modifications.The disclosures of all patents, patent applications and publicationscited herein are hereby incorporated herein by reference, to the extentthat they provide procedural or other details consistent with andsupplementary to those set forth herein.

1. A method of decreasing L*min and reducing stacked haze in a blackregion of an ink-jet printed image, comprising: ink-jetting at least oneink-jet ink onto an ink-jet ink receptive medium to form a printedmedium comprising an image having at least one black region, wherein atleast one said ink-jet ink includes a black ink-jet ink comprising: aliquid vehicle, and at least one black azo dye having the formula:

wherein X is SO₃O; R is unsubstituted C₁₋₄-alkyl; Q is K⁺, Na⁺ or Li⁺,or any combination of those; m is 1 or 2; and n is 1, 2 or 3; andstacking said printed medium, to obtain a printed medium havingdecreased L*min and reduced stacked haze in said at least one blackregion, relative to a printed medium printed with an ink-jet ink lackingsaid at least one black azo dye.
 2. The method of claim 1, wherein saidink receptive medium comprises a porous medium.
 3. The method of claim1, wherein one said black azo dye is 1H-Pyrazole-3-carboxylic acid,4-[[6-[[4-[(1,5-disulfo-2-naphthalenyl)azo]-2-methoxy-5-methylphenyl]azo]-5-hydroxy-7-sulfo-2-naphthalenyl]azo]-4,5-dihydro-5-oxo-1-(4-sulfophenyl)-lithiumsodium salt.
 4. The method of claim 1, wherein said black ink-jet inkcomprises said at least one black azo dye blended with at least oneother dye selected from the group consisting of black dyes, cyan dyes,magenta dyes and yellow dyes.
 5. The method of claim 4, wherein at leastone cyan dye has the formula

wherein M is a hydrogen atom or a metal atom or an oxide, hydroxide, orhalide thereof, P_(c) is a phthalocyanine nucleus, R¹ and R² are eachindependently a substituent selected from the group consisting of —SOX¹,—SO₂X¹, SO₂NX²X³, and —SO₃X⁴, wherein X¹, X², X³, and X⁴ are eachindependently a hydrogen atom, a substituted or unsubstituted alkylgroup, a substituted or unsubstituted aryl group, or a substituted orunsubstituted heterocyclic group, and wherein at least one of R¹ and R²has an ionic hydrophilic group as a substituent, and k and l are eachindependently an integer ranging from 1 to
 3. 6. The method of claim 4,wherein at least one said magenta dye is 6-Benzothiazolesulfonic acid,2-[4-cyano-3-(1,1-dimethylethyl)-5-[[4-methyl-6-[(6-sulfo-2benzothiazolyl)(2,4,6-trimethyl-3-sulfophenyl)amino]-2-[(2,4,6-trimethyl-3-sulfophenyl)amino]-3-pyridinyl]azo]-1H-pyrazol-1-yl].7. The method of claim 1, wherein at least one said yellow dye has theformula:

wherein R is hydrogen or an alkyl having from 1 to 6 carbon atoms; M ishydrogen or a metal atom or ammonium optionally substituted by alkyl,alkoxyalkyl or hydroxyalkyl; R₁₇, R₁₈ are independently hydrogen, alkylor alkoxy, each having from 1 to 3 carbon atoms; p₄ is from 1 to 3,wherein the sulfo group is in position 1, 4, 5, 6, 7 or 8 in the casewhere p₄ is equal to 1, or the sulfo groups are in positions 4 and 8, 5and 7, 6 and 8, or 1 and 5, in the case where p₄ is equal to 2, or thesulfo groups are in positions 3, 6 and 8 or 4, 6 and 8 in the case wherep₄ is equal to 3; and n and m are each independently from 2 to
 6. 8. Themethod of claim 1, wherein said at least one ink-jet ink comprises anink set including said black ink and at least cyan, magenta and yellowinks.
 9. The method of claim 8, wherein said cyan ink comprises a cyandye having the formula

wherein M is a hydrogen atom or a metal atom or an oxide, hydroxide, orhalide thereof, P_(c) is a phthalocyanine nucleus, R¹ and R² are eachindependently a substituent selected from the group consisting of —SOX¹,—SO₂X¹, SO₂NX²X³, and —SO₃X⁴, wherein X¹, X², X³, and X⁴ are eachindependently a hydrogen atom, a substituted or unsubstituted alkylgroup, a substituted or unsubstituted aryl group, or a substituted orunsubstituted heterocyclic group, and wherein at least one of R¹ or R²has an ionic hydrophilic group as a substituent, and k and l are eachindependently an integer ranging from 1 to
 3. 10. The method of claim 8,wherein said magenta ink comprises the magenta dye6-Benzothiazolesulfonic acid,2-[4-cyano-3-(1,1-dimethylethyl)-5-[[4-methyl-6-[(6-sulfo-2benzothiazolyl)(2,4,6-trimethyl-3-sulfophenyl)amino]-2-[(2,4,6-trimethyl-3-sulfophenyl)amino]-3-pyridinyl]azo]-1H-pyrazol-1-yl].11. The method of claim 8, wherein said yellow ink comprises a yellowdye having the formula:

wherein R is hydrogen or an alkyl having from 1 to 6 carbon atoms; M ishydrogen or a metal atom or ammonium optionally substituted by alkyl,alkoxyalkyl or hydroxyalkyl; R₁₇, R₁₈ are independently hydrogen, alkylor alkoxy, each having from 1 to 3 carbon atoms; p₄ is from 1 to 3,wherein the sulfo group is in position 1, 4, 5, 6, 7 or 8 in the casewhere p₄ is equal to 1, or the sulfo groups are in positions 4 and 8, 5and 7, 6 and 8, or 1 and 5, in the case where p₄ is equal to 2, or thesulfo groups are in positions 3, 6 and 8 or 4, 6 and 8 in the case wherep₄ is equal to 3; and n and m are each independently from 2 to
 6. 12.The method of claim 8, wherein said jetting of said at least one ink-jetink onto said medium comprises forming a composite image on said medium,wherein said composite image comprises said at least one black region.13. The method of claim 1, wherein said at least one black azo dyeimparts to said at least one black region a reduction in stacked hazeand a lower L*min, relative to the stacked haze and L*min of thecorresponding region(s) of an image printed with the same at least oneink-jet ink lacking said black azo dye.
 14. The method of claim 1,wherein the L*min of said at least one black region is between about4-12 on a CIELAB color space scale of 0-100.
 15. The method of claim 1,wherein said black ink comprises about 0.1% to about 10% of said atleast one black azo dye by weight of the ink.
 16. The method of claim15, wherein said black ink comprises about 1% to about 5% of said atleast one black azo dye by weight of the ink.